Virtual machines can be provided in a computer to enhance flexibility and utilization. A virtual machine typically refers to some arrangement of components (software and/or hardware) for virtualizing or emulating an actual computer, where the virtual machine can include an operating system and software applications. Virtual machines can allow different operating systems to be deployed on the same computer, such that applications written for different operating systems can be executed in different virtual machines (that contain corresponding operating systems) in the same computer. Moreover, the operating system of a virtual machine can be different from the host operating system that may be running on the computer on which the virtual machine is deployed.
In addition, a greater level of isolation is provided between or among applications running in different virtual machines. In some cases, virtual machines also allow multiple applications to more efficiently share common resources (processing resources, input/output or I/O resources, and storage resources) of the computer.
The computer typically has hardware memory protection mechanisms to isolate the virtual machines. However, software defects may allow one virtual machine to access the memory that belongs to another virtual machine, thereby compromising the isolation.